The "Student Preview" process is slightly different after the upgrade:

The "Go To Student View" button has been replaced by a graphic icon in the upper-right corner

While in Student Preview mode you'll see this bar at the top of the page

The "Return to Teacher View" button is replaced by the "Exit Preview" button - instructors can decide what to do with any data created while in Preview mode when they click "Exit Preview"

If any data was saved while exiting Preview mode the Student Preview icon turns green

Instructors use the same student preview account in each course, and multiple instructors in a course each have a different student preview account. Preview accounts display as Username_PreviewUser (i.e. Jsmith_PreviewUser) rather than "Demo User"